In a shipboard environment, placing a directional satellite antenna in a position to have a clear view of the sky in all directions is difficult. In some cases, large portions of the sky are blocked from the antenna's view by various obstructions (shipboard cranes, superstructure, etc.) necessitating the use of multiple antennas to ensure a clear view of the satellite at any heading.
There are several points at which the signal/data path can be split into the multiple paths. One such point is at the intermediate frequency (IF) level, between a single satellite modem and multiple transmit/receive paths. The signal path is shifted, either automatically or manually, from one path to another, much like a railroad switch shifts between sections of track.
The simplest means of shifting from one antenna to another is through the use of a mechanical switch. One consequence of mechanically shifting between antennas is that the signal, and hence the data stream, is briefly interrupted when the transition is made from one antenna to another. As with any mechanical device, there is a finite length of time when the switch is between two states. This “in between time” is rather long in comparison to the signal period of the signal being switched.
One way of mitigating this interruption in the signal is through the use of buffers, either internal or external to the modem. In either case, the ability to empty the transmitting buffer and fill the receiving buffer requires the use of additional bandwidth on the physical layer or throttling back the data rate to the transmitting modem. In the case where the buffering is internal to the modem, this solution is highly proprietary, specific to a particular modem pair. External buffering independent of the satellite modem is possible. This would require complex external electronics, most likely microprocessor-based, to manage the bandwidth available between the serial device and the satellite modem.
Another solution is to maintain the connection between the two antennas continuously. The use of multiple antennas concurrently is difficult due to differences in path length resulting in phase differences in the incoming signals. In some cases, phase-shifting circuitry is used to phase-shift the incoming signals to correct for differences in signal path length and prevent out-of-phase conditions from degrading the resulting signal. Such phase-shifting circuitry can be costly in terms of expense and complexity.
Another solution illustrated in FIG. 1. In this configuration, the availability of only one antenna is used as the selection criteria for an antenna handoff. One antenna controller controls an electronic “a/b” switch which selects which antenna to use. This causes handoffs to occur unnecessarily, such as when antenna “a” comes out of a programmed blockage sector. In this scenario, the switches would shift back to antenna “a” even if antenna “b” still had a clear view of the satellite.